Fa Lecture Note

Failure Analysis & Accident Investigation S.K. Bhaumik Failure Analysis & Accident Investigation Group, Materials Science Division National Aerospace Laboratories, Bangalore 560 017 E-mail: [email protected] 1. Introduction Failures and accidents are unavoidable events in the service life of any engineering component or system, however small the probability is. While failure is an undesirable event in the service life of an ordinary component or structure, an aircraft or industrial accident is totally unacceptable since, unlike the former where it is more of an inconvenience, the latter may also lead to loss of life. While it is desirable to analyse all failures, it becomes mandatory to analyse failures and accidents in critical, high technology areas. Systematic failure investigation uncovers the cause or causes leading to the failure of the component so that remedial measures can be initiated to prevent the recurrence of similar failures. Failure analysis, therefore, leads to better reliability of the component or system. Failure represents an adverse situation wherein a component or assembly fails to perform its intended function satisfactorily. In other words, it can be defined as the gap between the expected performance and the actual performance of any component or assembly. Failure does not necessarily involve fracture always. For example, excessive elastic deformation of a component in an assembly can interfere the functioning of other components leading to failure. Therefore, the excessive elastic deformation of the component is termed as the failure and this can be because of various reasons such as improper selection of material, improper design, excessive load etc. Failure analysis can be defined as the examination of a failed component and of the failure situation in order to determine the causes of the failure. The purpose of failure analysis is to establish the mechanism and causes of the failure and to recommend a solution to the problem. Since most of the time, failures are “caused”, they do not “just happen”, identification of the cause (s) for the failure helps to prevent recurrence of similar failures. Since even the most sophisticated simulation testing cannot adequately duplicate the varied factors and the many unanticipated causes that lead to failure, failure analysis offers the most reliable tool in assuring the safety of the component. 2. Failure Analysis Methodology The importance of analyzing failures has already been highlighted. There is a great need to carry out the analysis in a proper methodical manner so that no important fact is overlooked, and evidences are not lost during the analysis. Though the approach to failure analysis and the methodology are often governed by the expertise and knowledge of the analyst, some general guidelines may be given for efficient conduct of the investigation. Whenever an investigation is to be carried out, at the out set, it is essential to gather relevant background data. This facilitates the development of a complete case history about the failure. The information to be collected falls into two categories: (a) information about the failed component and (b) information about the failure itself. Wherever possible, the investigator should visit the site of the failure/accident and make a firsthand examination and estimation of damages. Photography is the best method for recording damages and videographing the removal operations is immensely useful.

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The investigation itself should be properly planned. The various tests to be carried out must be judiciously selected and the sequence of tests should be carefully planned. Decisions on any tests that would involve destruction of part of the component should be taken very carefully. A wrong sequence of tests may destroy some of the important evidence or introduce features, which could be confusing. 2.1 Specimen collection Collection of specimens for further laboratory examination is a crucial step in any failure analysis or accident investigation. This has to be done very judiciously. Wrong choice can, apart from wasteful work, lead to confusion and wrong direction of investigation. Primary failure must be distinguished from numerous secondary failures. Samples from the suspected primary failure region must be carefully collected and their location in the wreckage and in the original structure must be recorded. Other samples, which can provide secondary evidence, must also be collected. The fracture surfaces must be very carefully handled as they can provide a fund of useful information about the mode and mechanism of fracture during detailed laboratory examination. In the field, it is better to keep the samples in plastic covers with suitable desiccants and sealing them and securing the bags with suitable identifying tags. The fracture surfaces may be sprayed with a transparent lacquer before sealing. Touching the fracture surfaces must be avoided as the human sweat is a corrodant. The mating surfaces of a fracture should never be made to touch each other as this would cause abrasion and thereby lead to loss of vital microfractographic evidences. Rough treatment or the formation of corrosion products on the fracture will obscure vital information. Education in the proper handling of specimens prior to any fractographic examination is strongly recommended for anyone dealing in fractures either in the field or in the laboratory (Fig.1). 2.2 Preliminary examination Visual examination of the component and the fracture surface immediately reveals the nature of fracture, presence of adhering debris and corrosion products, change in surface colour, abrasion and rub marks, metal slivers and burrs sticking out, etc. This examination also indicates the quality of workmanship in the manufacture of the component and any abuse which might have experienced during service. The preliminary examination of the fracture surface is best carried out by naked eye or with a low power microscope. Documentation at this stage must be completed with the help of sketches and macrophotographs. 2.3 Microscopic examination Detailed examination of the fracture surfaces should be carried out at various levels of magnification and resolution using optical and electron microscopes. These should be supplemented by metallographic examination of selectively chosen sections of the component. The fund of information that can be generated from these tests is phenomenal and is extremely useful to pinpoint the cause, mechanism and sequence of the failure event. 2.4 Chemical analysis Chemical analysis of samples from the component provides information regarding any deviation from the standard specifications, compositional inhomgeneties, impurities,

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inclusions, segregation, etc. It also helps in identifying the nature of corrosion products, coatings, external debris, etc. Analysis at microscopic levels provides information about the nature of inclusions, phases, and surface layers. Several cases of service failures are known to have been caused by the presence of deleterious inclusions from which cracks start in the component and propagate leading to fracture. Certain impurities are known to cause embrittlement in metals. Segregation of constituent elements sometimes provide easy path for crack propagation. Hence identification of these harmful constituents is very important in failure analysis. A variety of instruments are available for bulk chemical and micro-chemical analysis. 2.5 Mechanical properties Evaluation of the mechanical properties of the component is an important step in any failure analysis. This enables the investigator to judge whether the material with which the component is made meets the specifications and whether the component was capable of withstanding the service stresses. If the size of the failed component permits, samples can be taken from the component and the conventional mechanical testing be done by standard test procedures. Test for tensile properties is generally the most useful one in many cases. Other properties such as impact strength, toughness, and creep rupture provide clues for the mechanism of failure. Sometimes, even tests on miniature specimens would provide vital information. If the condition of the component does not permit tensile or other mechanical tests, even a hardness measurement would help in estimating the tensile strength. In some situations, evaluation of the mechanical properties of components that have not yet failed but are still in service may be necessary. 2.6 Non-destructive evaluation Failures are the end results of the crack originating in the component from flaws which were already existing or which formed during service. Non-destructive evaluation (NDE) is employed to detect at an early stage subsurface flaws and internal flaws in the component, their type, size, orientation and location. In a failed component, there may still be flaws similar to the one that was primarily responsible for the failure. These flaws can be detected by NDE methods. Also, flaws in similar components from the same batch as the failed one can be detected so that their use can be avoided or restricted. Various techniques are available for examining a component for flaws without actually destroying it. 2.7 Simulation studies Quite often it is helpful to test the component under simulated service conditions. Such studies throw light on the suitability or otherwise of the material of construction, the adopted design and the processing history of the component in question. If components from the same batch as that of the failed component did not fail during simulation tests, then the cause of the failure is singular to the particular failed component. Also, examination of a similar component in service, which has not failed, would provide additional useful information. The simulation test paves the way for further investigation into other possible causes.

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2.8 Analysis of data The most important task in any failure analysis is the consolidation and systematic connection of all the data obtained in the tests described above. For every failure, all the tests described may not be necessary. The results of tests must be compared against the specifications and deviations if any should carefully be considered as possible contributing factors. At this stage, expertise from other related disciplines would be very useful in interpretation of data. A fund of information on various aspects of the failure would become available through proper analysis. These include the failure initiation site or sites, crack length, its propagation path and speed, and the nature and direction of load acting on the component. The role of other factors such as temperature, corrosion, wear, component manufacturing history, assembly and alignment, also become clear during the analysis. Finally, it must be born in mind that in some cases, it may not be possible to specify the cause or causes of the failure with certainty. At the most, it must be stated clearly which conclusions are based on determined facts and which are based on conjecture and circumstantial evidences. 3. Types of Failures and their Identification There are enumerable types of failures that can occur in service components. However, they can be broadly categorized in three major types of failures that are of engineering importance. Briefly, the main features in these types of failures are described. 1. Mechanical failures •

Ductile and brittle failures

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Fatigue failures

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Distortion failures

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Wear failures

2. Environmental failures

3. Mechanical-environmental failures

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Corrosion failures

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Stress-corrosion cracking

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Corrosion-erosion failures

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Hydrogen embrittlement

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Liquid metal embrittlement

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Corrosion fatigue

3.1 Mechanical failures Ductile and brittle failures Ductile

Brittle

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They are characterized by tearing of metal accompanied by gross plastic deformation.

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They are characterized by rapid crack propagation without appreciable plastic deformation.

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Macroscopically they have a gray, fibrous appearance and exhibit necking (Fig.2a).

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Macroscopically they have a bright, granular appearance and exhibit little or no necking (Fig.2b).

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Ductile fractures are classified on a macroscopic scale as flat (normal to the maximum tensile stress) or shear (at 450C to the maximum tensile stress). Microscopically they occur by microvoid formation and coalescence. These fractographic features are normally referred to as dimpled rupture (Fig.3)

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Brittle fractures are usually of the flat type (normal to the direction of maximum tensile stress).

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They occur by either transgranular (cleavage or quasicleavage or intergranular cracking) (Figs.4).

Fatigue failures Fatigue accounts nearly 80% of the total service failure and has great significance in design of any component. Fatigue is the progressive, localized, permanent structural change that occurs in a material subjected to fluctuating stresses or strains. Fatigue fracture surfaces have typical appearance in which a half-moon shaped region consisting of crack arrest marks or more commonly known as beach marks are seen (Fig.5). Similar macroscopic features can also be seen in other progressive mode of fracture such as hydrogen embrittlement or stress corrosion cracking. However, microscopic features for each of these fractures are distinct and are easily distinguishable. For example, as the fatigue crack propagates, it usually leaves on the fracture surface, behind the advancing crack front, regions of depression and elevation. These are known as fatigue striations. Presence of striations on the fracture surface is conclusive evidence that the component has failed by fatigue. The characteristic microscopic features of hydrogen embrittlement and stress corrosion cracking will be discussed later. The process of fatigue consists of following three stages: 1. Initial fatigue damage leading to crack initiation 2. Crack propagation 3. Final sudden fracture Distortion failures Distortion failure occurs when a structure or component is deformed either elastically or plastically. It can be either size distortion or shape distortion. Special type of distortion Ratcheting: Ratcheting is strain-dependent phenomenon wherein one or more of the over-all dimensions of a member or structure change relatively uniformly along the direction of steady state stress due to cyclic accumulation of plastic strain. Ratcheting is normally observed when a component is stressed by steady state loading, either unaxial or multiaxial, with a varying strain in a direction other than the direction of principal stress. Wear failures Wear can be defined as the damage to the surface caused by the removal or displacement of material by the mechanical action of contacting solid, liquid or gas.

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Various types wear that are normally encountered by the service components are adhesive wear, abrasive wear, erosive wear, corrosive wear, surface fatigue, liquid erosion and fretting. Adhesive wear is transference of material from one surface to another during relative motion due to a process of solid-phase welding. This is also called variously as scoring, galling, seizing and scuffing. Abrasive wear or abrasion is displacement of material from a solid surface by contact with hard particles on a mating surface or with hard particles that are moving relative to the wearing surface. All types of abrasive wear involve basically same mechanism, namely, penetration and subsequent grooving of the surface by abrasive particles. Erosion or erosive wear is the loss of material from a solid surface due to relative motion in contact with a fluid that contains solid particles. In abrasive erosion the solid particles move nearly parallel to the solid surface whereas, in impact erosion the relative motion of solid particles is nearly normal to the solid surface. The erosion of material can be attributed to a number of mechanisms including cutting, plowing, extrusion, fragmentation, elastic fracture etc. Fretting is a wear phenomenon that occurs between two mating surfaces. It is adhesive in nature and vibration is its essential causative factor. Fretting is accompanied by corrosion. Common sites for fretting are joints that are bolted, keyed, pinned, press-fitted and riveted, oscillating bearings, splines, couplings, spindles and seals, and universal joints. Fretting damage occur in three stages: (i) initial adhesion, (ii) oscillation accompanied by the generation of oxidized debris followed by (iii) fatigue and wear in the region of contact. 3.2 Environmental failures Corrosion is the unintended destructive chemical or electrochemical reaction of a material with its environment. Corrosion can lead to service failure of metal parts or render them susceptible to failure by some other mechanism. Corrosion generally leads to gradual loss of material from the component leading to failure. In some cases, it facilitates the failure of the component by some other mechanism. For example, pitting corrosion can lead to stress concentrators resulting in fatigue crack initiation. Aluminium alloys of the series 2000 (AlCuMg), 5000 (AlMg), and 7000 (AlZnCuMg) in the rolled and extruded conditions are used extensively as aircraft structural members. These alloys are susceptible to exfoliation corrosion in marine and industrial environments. Exfoliation corrosion is a special form of corrosion in which the intergranular corrosion proceeds selectively along the grain boundaries of the elongated grain structure. The corrosion products lift thin layers of uncorroded metal from the surface. And these subsequently suffer corrosion themselves producing a typical layered appearance of the corrosion product. 3.3 Mechanical-environmental failures The surroundings or conditions – physical, chemical and mechanical – in which a material exists, has profound influence on its performance. Environment assisted fracture of engineering materials, also known as mechanical-environmental failures, refer to cracking of engineering metals and alloys under the conjoint action of stress and a corrosive environment. Environment assisted fracture of engineering materials has an enormous impact in many diverse segments of the economy such as power generation,

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chemical, petroleum, petrochemical and mineral processing, transportation etc. and is frequently the limiting factor to advancing the state of the art. Stress corrosion cracking and hydrogen embrittlement, the most common forms of mechanical – environmental failures, have attracted the attention of engineers and metallurgists for a considerable time not merely because of their common occurrence but also because of the somewhat common characteristics they exhibit. Stress corrosion cracking (SCC) Stress corrosion cracking is a mechanical-environmental failure process in which synergistic action of sustained tensile stress and corrosive environment combine to initiate and propagate fracture in a metal part. Failure by stress corrosion cracking is frequently encountered in seemingly mild chemical environments at tensile stresses well below the yield stress of the metal. The failure often takes the form of fine cracks that penetrate deep into the metal with little or no evidence of corrosion on the near by surface. The general characteristics of SCC are •

only specific environments contribute to this type of failure for a given alloy

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high purity metals are much less susceptible than most commercial grades of metals and alloys

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certain aspects of the metallurgical structure of an alloy influence its susceptibility to SCC

Hydrogen embrittlement (HE) Hydrogen embrittlement can be broadly defined as a reduction in the ductility of a metal by the presence or absorption of hydrogen. Many diverse hydrogen embrittlement problems arise due to pick up of hydrogen in process and fabrication or in service in a hydrogenous environment. Generally, one may distinguish among three types of problems. Problems in melting, problems in processing and fabrication (welding, electroplating) and problems in service. The general characteristics of hydrogen embrittlement are •

hydrogen embrittlement can be reversible or irreversible

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there is a lower critical (applied or residual) stress below which the delayed failure does not occur. The lower critical stress is sensitive to lattice dissolved hydrogen, microstructure, applied stress and yield strength

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hydrogen embrittlement is sensitive to strain rate and temperature. Briefly, the embrittlement (loss of ductility) is enhanced by slow strain rates and moderately elevated temperatures

4. Common Causes of Failures 4.1 Design errors Failures in components often result from inadequacies in design, even with the use of the best material of construction. Some of them result from design deficiencies of a nature indicating that little engineering effort was made to avoid design features known to be conducive to failure. At the other extreme, sometimes even a carefully conceived

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and thoroughly evaluated design may still be deficient and contribute to early failure in service. Ad-hoc modifications without proper design review also contribute to a number of failures. 4.2 Manufacturing errors Defects introduced during various manufacturing stages can have serious weakening effect on the component. They can also nucleate cracks that can then propagate by fatigue, leading to premature failures. 4.3 Assembly errors With the best component design and choice of the best material, sometimes mistakes can happen during assembly. Such assembly errors are often not detected during inspection and usually, they do not prevent apparently normal operation. Deficiencies of this type are generally related to inaccurate, incomplete or ambiguous assembly specifications, but they also occur as a consequence of human error or negligence. 4.4 Inspection •Wrong technique •Non-calibration of equipments 4.5 Operation Failures are also caused by abuses in service. Many a failure has been witnessed when products were abused without recognizing the serious consequences. •

Inadequate lubrication

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Inadequate cleaning

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Nonadherence to inspection schedule Wrong application

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Nonconformity with operating instructions Over speeding High operating temperatures Wrong operating tools Used in environment not Overloading

4.6 Maintenance Service failure due to improper maintenance accounts for sizable fraction of total failures in engineering/aerospace industries. In spite of strict maintenance schedule and procedure, there have been instances wherein inadequate attention in following the methodology/procedure and/or use of tools led to premature failure of components. . 5. Questions that a failure analyst need to answer

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After completion of all the procedures to the extent dictated by individual failures, the analyst is ready to interpret and summarize the facts that he has gathered. Most failures are caused by more than one factor, though frequently one factor may predominate. Therefore, after collecting the evidences, it would be relevant to pose and answer the following questions before formulating the conclusions. •

Did the failure involve cracking or fracture?

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Was the crack initiation surface or subsurface?

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Was the cracking associated with a stress concentrator?

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How long the crack was present?

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What was the type of loading – static, cyclic or intermittent?

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What was the failure mechanism?

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Was the proper material used?

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Was the component that failed properly fabricated, heat treated and assembled?

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Was the failure related to abuse in service?

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Are failures likely to occur in similar components in service? If so, what can be done to prevent their recurrence?

6. Conclusions It is now well recognized that failures do not just happen, but are caused. Hence, failure analysis assumes supreme importance. There are compelling reasons for investigating failures. Unless the true cause of the failure is known, no remedial action can be initiated. Failure analysis helps a lot in improving the reliability and safety of machinery and structures, which form the heart of modern industries. In the last few decades, systematic investigations carried out on many failed components and structures have generated a fund of useful information for taking suitable remedial measures to prevent recurrence of failures and accidents. The lessons learned from failure analysis are vital for the engineering profession and the industries that aim at design and manufacture of products with the probability of service failure at the absolute minimum.

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Figure 1. Fracture surfaces are like fingerprints; handle them with care

Figure 2. Fracture surface of two tensile samples: (a) ductile fracture, with flat fracture in the center and slant fracture on the sides (cup and cone fracture) and (b) brittle fracture

Figure 3. SEM fractograph of the of ductile fracture surface; dimples due to formation of voids and coalescence

Figure 4. SEM fractograph of brittle fracture surface showing typical of intergranular fracture

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Figure 5(a). Fatigue fracture; macroscopic features (beach marks)

Figure 5(b). Fatigue fracture; microscopic features (fatigue striations)

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